Fix missing focus ring on all buttons and button groups (#2920)

.changeset/fix-button-focus-ring.md

@@ -0,0 +1,5 @@
+---
+"@tabler/core": patch
+---
+
+Fixed the missing focus ring on buttons and `btn-check` button groups by using the shared focus ring token.

core/scss/_variables.scss

@@ -1118,14 +1118,13 @@ $btn-border-width: $input-btn-border-width !default;
 
 $btn-font-weight: var(--font-weight-medium) !default;
 $btn-box-shadow: var(--shadow-input) !default;
-$btn-focus-width: $input-btn-focus-width !default;
+$btn-focus-box-shadow: $focus-ring-box-shadow !default;
 $btn-disabled-opacity: 0.4 !default;
 $btn-active-box-shadow: inset 0 3px 5px rgba($black, 0.125) !default;
 
 $btn-link-color: var(--link-color) !default;
 $btn-link-hover-color: var(--link-hover-color) !default;
 $btn-link-disabled-color: $gray-600 !default;
-$btn-link-focus-shadow-rgb: to-rgb(color.mix(color-contrast($link-color), $link-color, 15%)) !default;
 
 $btn-border-radius: var(--border-radius) !default;
 $btn-border-radius-sm: var(--border-radius-sm) !default;

core/scss/bootstrap/_buttons.scss

@@ -20,7 +20,10 @@
   --btn-hover-border-color: transparent;
   --btn-box-shadow: #{$btn-box-shadow};
   --btn-disabled-opacity: #{$btn-disabled-opacity};
-  --btn-focus-box-shadow: 0 0 0 #{$btn-focus-width} rgba(var(--btn-focus-shadow-rgb), 0.5);
+  // Zero-size shadow, not `none`: the active state is combined with the focus
+  // ring in one `box-shadow` list, where `none` would negate every other shadow
+  --btn-active-shadow: 0 0 0 #000;
+  --btn-focus-box-shadow: #{$btn-focus-box-shadow};
   // scss-docs-end btn-css-vars
 
   display: inline-block;
@@ -172,7 +175,6 @@
   --btn-disabled-color: #{$btn-link-disabled-color};
   --btn-disabled-border-color: transparent;
   --btn-box-shadow: 0 0 0 #000; // Can't use `none` as keyword negates all values when used with multiple shadows
-  --btn-focus-shadow-rgb: #{$btn-link-focus-shadow-rgb};
 
   text-decoration: $link-decoration;
   @if $enable-gradients {